Time Machine External Hard Disk with Zero Bytes for most folders, Size unchanged

Hi,

I need to know if anyone can help with this difficult issue; I'd be thankful for any assistance. Maybe someone has experienced a similar problem, a situation which I think is in this case unrelated to the external hard disk being formatted for Windows. Unfortunately I can't recall what the format was when it arrived, but I know it was correctly reformatted. It's 3 years old, I believe, Toshiba, but that's all I really know.

This is extremely complicated, I'm afraid, because even though I've categorised this as a Monterey problem, it only happened after I downgraded to Catalina. I have a 2020 Macbook Pro. The downgrade involved a lot of typing but seemed successful... and then this happened. Stupidly, this external hard disk is my only backup disk and it's also the Time Machine backup. At first, the whole disk was zero bytes (size unchanged, so there remained as much used space as there really is on the disk) and I did not have permission to open "Backups.backupdb". There's plenty of room on the disk, btw, so that's not what's wrong.

Then I hooked the disk up to an older Macbook (not mine) and it was fine, all the permissions were normal and all the folders were back and they were functional. When I retried it with my 2020 computer, it looked like all the backups were there, except that the two oldest ones were unopenable and Zero Bytes. So, as it turned out, were almost all the folders inside of each of the openable backups. They either have a minus sign or an down-arrow, and "The folder 'Whatever' can't be opened because you don't have permission to see its contents".

Sorry for the lengthy explanation. Please offer advice if you can.